The present invention relates to a color image processing apparatus and, more particularly, to a color image processing apparatus for automatically feeding one at a time a stack of a plurality of documents to process images printed thereon.
Color copiers available today include one of the type having a single image reproducing unit and reproducing a full-color image by repeating an iterative copying process a plurality of times. Specifically, this type of copier sequentially executes a copying process in each of four fundamental colors, i.e., cyan (C), magenta (M), yellow (Y), and black (BK) (or only C, M and Y), the resultant color components being transferred one above another to a single recording sheet. In a monochromatic mode, the copier reproduces a single image by a single copying process. The operator, therefore, has to attend to the switchover of the color mode because the required copying time greatly differs from the full-color mode to the monocolor mode, i.e., the operator has to switch over the color mode depending on the kind of a copy.
To free the operator from such troublesome mode switching operations, the operation mode of a copier may be switched over automatically by automatically determining whether a document image is color or black-and-white, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 63-107274. The apparatus taught by this Laid-Open Publication compares the densities of Y, M and C of an image signal from which an achromatic component (black) has been removed (output of an undercolor removing circuit) with a predetermined threshold value, and determines that the image is achromatic or black-and-white if all the densities of Y, M and C are extremely low. Alternatively, the discrimination of an image as to color/black-and-white may be effected by extracting, among color components C, M and Y of an image signal, the maximum and minimum values and producing a difference therebetween, as shown and described in Japanese Patent Laid-Open Publication (Kokai) No. 62-101179.
A problem with the prior art apparatuses discussed above is that the accuracy of discrimination of an image as to color/black-and-white (achromatic color) is too low to eliminate discrimination errors. Should the threshold value with which image data should be compared be increased to reduce discrimination errors, all the thin image components would be determined to be achromatic.
The present invention contemplates to automatically discriminate an image as to color/achromatic with high accuracy and thereby executes optimal image processing matching the kind of an image without resorting to operator's manipulations. To this end, the present invention provides a color image processing apparatus comprising image reading means having spectral filter means for reading a visible image printed on a document sheet in a predetermined image reading area by scanning the image, color discriminating means for automatically determining whether or not the image is substantially monochromatic by processing image data produced by the image reading means and separated into a plurality of fundamental colors, edge detecting means for detecting edges of the image by processing the image data, discrimination inhibiting means for inhibiting the color discriminating means from operating when the edge detecting means has detected an edge of the image, and control means for automatically selecting and executing either one of color processing and monochrome processing in response to a result of discrimination by the color discriminating means.
The present invention determines whether or not a document image is monochromatic (especially achromatic) by processing image data derived from the document. In this type of apparatus, image reading unavoidably suffers from the deterioration of MTF (Modulation Transfer Function) due to the influence of a lens included in optics, for example. Specifically, edge portions of a document image where the tone and/or color sharply changes are blurred in a reproduction because the tone and/or color of the edge portions changes slowly in image data. Further, this kind of degradation of image quality appears with a different characteristic in each of the color-separated image components (red (R), green (G) and blue (B)). Moreover, sensors each being assigned to respective one of color-separated image light (R, G and B) are not free from positional deviations, so that the color components R, G and B in image data involve phase deviations corresponding to the positional deviations of the sensors.
The difference in MTF characteristic and the phase deviation of the color components stated above directly translate into color deviations in image data. For example, despite that only black (achromatic color) produced by combining R, G and B exists on a document image, image data representative of the document image is partly short of the color component R, G or B due to the deviation of R, G or B with respect to position (or time). Then, such a part will turn out to be chromatic. Thus, discriminating an image as to chromatic/achromatic by processing image data would render colors different from the colors of a document image. Nevertheless, it is only in the edge portions of an image where the tone and/or color sharply changes that erroneous discrimination is apt to occur. In accordance with the present invention, while image data representative of a document image is processed, the color discrimination is inhibited when an edge of the document image is detected. This is successful in minimizing erroneous discrimination and thereby allowing an image to be discriminated as to color/monochrome with high accuracy.
By using a signal produced by removing an achromatic (BK: black) component from image components Y, M and C, it is possible to promote easy discrimination of an image as to color/monochrome. Usually, this kind of signal is generated by an undercolor removing (UCR) circuit incorporated in an image processing unit. Therefore, the circuit arrangement will be simplified if the existing circuit is also usable for color/monochrome discrimination.
To simplify the circuit arrangement for the discrimination, the present invention provides a color image processing apparatus comprising image reading means having spectral filter means for reading a visible image printed on a document sheet in a predetermined image reading area by scanning the image, image processing means interconnected to an output of the image reading means, color image recording means interconnected to an output of the image processing means and comprising a plurality of chromatic recording systems which are different in hue from each other and an achromatic recording system, undercolor removing means included in the image processing means for processing input image data to detect an achromatic color component contained in the image, extracting as achromatic color data a value of a predetermined ratio associated with the detected achromatic color component, and subtracting a value corresponding to the extracted achromatic color data from the input image data to output chromatic color data, color discriminating means for determining whether or not the image is substantially monochromatic by processing the chromatic color data outputted by said undercolor removing means, edge detecting means for detecting edges of the image by processing the image data, discrimination inhibiting means for inhibiting the color discriminating means from operating when the edge detecting means has detected an edge of the image, and control means for causing the image reading means to read a single document image a plurality of times, the control means fixing, during a first image reading operation, the ratio of the undercolor removing means substantially to 100% and inhibiting the color image recording means from operating, the control means automatically and selectively conditioning the color image recording means to either one of a color processing mode and a monochromatic processing mode in response to a result of discrimination which the color discriminating means produces with image data resulted from the first image reading operation.
In the above construction, each document image is simply read and not recorded at first (prescanning) and, at this instant, the color discriminating means determines whether or not the image is monochromatic. During the second and successive image readings, recording is executed in color or monochrome. Since the removing ratio of the undercolor removing means is set to 100% during the prescanning, chromatic data outputted by the undercolor removing means substantially does not include an achromatic component. When such data is compared with a particular threshold value, whether or not the image is monochromatic will be readily determined.
In the event of actual image recording, 100% UCR processing would introduce a black component over the entire recorded image and thereby make it difficult to match C, M and Y ink and black ink. Nevertheless, the above construction of the present invention needs 100% UCR processing only during prescanning and sets an adequate ratio in the undercolor removing circuit during the second and successive image processing, promoting easy matching of C, M and Y ink and black ink in a recorded image with respect to tone. Hence, the undercolor removing circuit can be shared by the color/monochrome discrimination and the generation of a black component for image recording, whereby the circuit arrangement is simplified.
When prescanning is effected as stated above, even monochromatic image recording needs two times of scanning and, therefore, a substantial processing time. With a popular inexpensive image processing apparatus which lacks a frame memory, it is difficult to eliminate prescanning because such an apparatus is substantially not capable of executing image reading and recording at the same time.
To eliminate prescanning and thereby reduce required processing time, the present invention provides a color image processing apparatus comprising image reading means having spectral filter means for reading a visible image printed on a document sheet in a predetermined image reading area by scanning the image, image processing means interconnected to an output of the image reading means, color image recording means interconnected to an output of the image processing means and comprising a plurality of chromatic recording systems which are different in hue from each other and an achromatic recording system, background removing means included in the image processing means for correcting tone of input image data and removing a set background level from the image data, undercolor removing means included in the image processing means for processing image data outputted by the background removing means to detect an achromatic color component contained in the image, extracting as achromatic color data a value of a predetermined ratio associated with the detected achromatic color component, and subtracting a value corresponding to the extracted achromatic color data from the input image data to output chromatic color data, color discriminating means for determining whether or not the image is substantially monochromatic by processing the chromatic color data outputted by the undercolor removing means, edge detecting means for detecting edges of the image by processing the image data, discrimination inhibiting means for inhibiting the color discriminating means from operating when the edge detecting means has detected an edge of the image, and control means for setting, when the image reading means reads a single document image for the first time, a first background level in the background removing means, fixing the ratio of the undercolor removing means at substantially 100%, and in this condition causing the color image recording means to record an achromatic color component of the image, the control means setting, if a result of discrimination by the color discriminating means resulted from the first image reading operation is not monochromatic, a second background level in the background removing means, setting the ratio of the undercolor removing means such that the ratio is zero if tone is lower than a tone substantially the same as the first background level and has a predetermined value if otherwise, and in this condition executing a second and successive reading operations while conditioning the color image recording means for a recording mode for recording chromatic color components.
The above construction has the following device in order to eliminate prescanning. To begin with, during the first scanning, an achromatic component (black) is recorded at the same time as image reading. The undercolor removal (UCR) ratio is set to 100%. The background removing means removes an image component whose density is lower than the first background level in order to prevent an image area lower in density than the first background level from being recorded. Usually, when the 100% UCR ratio is selected, black ink will be recorded over the entire image area and make it difficult to match C, M and Y ink and black ink. Nevertheless, the above construction applies undercolor removal to data from which the background component of an image has been removed, so that black ink is recorded only in the portions where the density is higher than the background level, as in recording in black implemented by a skeleton black recording method known in the art. This allows black ink and C, M and Y ink to be readily matched to each other. Further, the resultant chromatic data has undergone 100% UCR processing and facilitates the discrimination by the color discriminating means.
When the image is determined to be monochromatic by the first scanning, the recording operation can be ended immediately because the recording in black has already been completed. In this case, density components lower than the first background level will not appear in a recorded image. This is rather favorable considering the fact that a monochromatic image often contains only characters and, therefore, has noticeable changes in density, i.e., removing the background erases noise image and leaves only necessary data. When the image is not monochromatic as determined by the first scanning, the second and successive scanning are executed while image data are recorded in chromatic ink of different colors (C, M and Y). For the second and successive scanning, the background level is changed to zero to stop the removal of background, while the undercolor removing means removes the undercolor at a predetermined ratio. Concerning this ratio, zero is selected for densities lower than the first background level while an adequate ratio other than zero is selected for higher densities. More specifically, the background removing level for the first scanning (first background level) and the boundary level for the switchover of the undercolor removal for the second and successive scannings are selected to be the same as each other. Hence, an image component lower in density than the first background level and, therefore, not recorded in black ink during the first scanning is recorded to 100% in chromatic ink during any of the second and successive scanning.
With the apparatus disclosed in the previously stated Laid-Open Patent Publication No. 63-107274, the operator has to position the individual documents in a document reading area every time copying processing is to be executed. An automatic document feeder (ADF) is available for automatically and continuously processing images which are printed on a number of document sheets. An ADF feeds one at a time a stack of document sheets loaded on a tray so as to locate them in an image reading area, the uppermost document sheet being first. This frees the operator from the manual replacement of documents and implements continuous automatic copying. In this kind of conventional ADF, a document feed control signal is generated in order to replace a document every time the document reading operation is repeated n consecutive times which corresponds to the desired number of copies of that document.
However, when the stack of documents on the tray include both of monochromatic (e.g. black-and-white) documents and color documents, they cannot be reproduced by fully automatic processing despite the use of an ADF. Specifically, unless the operator sets up either one of the color and monochromatic modes every time the kind of a document image (black-and-white or color) changes, all the documents will be dealt with in the color mode or the monochromatic mode. While the operator may separate a desired stack of documents into color documents and monochromatic documents beforehand in order to minimize the number of times of the switchover, such an operation is time- and labor-consuming.
The present invention allows, even when a document stack has both of color and monochromatic document, the documents to be processed continuously without resorting to the operator's manipulations while minimizing the processing time. For this purpose, a color image processing apparatus of the present invention comprises image reading means having spectral filter means for reading a visible image printed on a document in a predetermined image reading area by scanning the image, color discriminating means for automatically determining whether or not the image is substantially monochromatic by processing image data outputted by the image reading means and separated into a plurality of fundamental colors, edge detecting means for detecting edges of the image by processing the image data, discrimination inhibiting means for inhibiting the color discriminating means from operating when the edge detecting means has detected an edge of the image, automatic document feeding means loaded with a plurality of document sheets each carrying a visible image thereon for feeding the document sheets one by one to the predetermined image reading area of the image reading means, and control means for switching over, on the basis of a result of discrimination by the color discriminating means, correspondence between the number of times that the image reading means scans a document sheet and the number of times that the automatic document feeding means feeds a document sheet.
In the above construction, the color discriminating means detects the color of an image printed on a document and lying in the image reading area and, at the same time, automatically determines whether or not the color is substantially monochromatic. Based on the result of discrimination, the control means switches over the correspondence between the number of times that the image reading means scans a document and the number of times that the automatic document feeding means feeds a document. For example, in a color copier selectively operable in a full color mode which produces a single copy by repeating a copying process four consecutive times and a black-and-white mode which produces a single copy by a single copying process, every time a new document is fed, whether or not an image printed thereon is color or black-and-white is discriminated automatically. If the image is a color image, it is scanned 4.times.n (desired number of copies) times, and then the next document is fed. If the image is a black-and-white image, it is scanned n times, and then the next document is fed. Hence, even when color documents and black-and-white documents are stacked together, the apparatus automatically discriminates them as to color and feeds them at adequate timings. This allows the documents to be continuously processed within a minimum necessary period of time while freeing the operator from troublesome manipulations. When an image is printed on a document in ink of single hue such as C, M or Y, it can be processed by single scanning and, therefore, handled in the same manner as a black-and-white image.